The present invention relates to a method and apparatus of forming a seamless cellulose tube, suitable for use as a food casing, using a solution of nonderivatized cellulose, tertiary amine N-oxide and water.
Cellulose food casings are well known in the art and are widely used in the production of stuffed food products such as sausages and the like. Cellulose food casings used in the manufacture of small diameter sausages such frankfurters and the like generally are seamless tubes formed of a regenerated cellulose and contain aplasticizer such as water and/or a polyol such as glycerine. Plasticization is necessary because otherwise the cellulose tube is too brittle for handling and commercial use.
Cellulose food casings of pure regenerated and non-reinforced cellulose for the manufacture of frankfurters generally have a wall thickness ranging from about 0.025 mm to about 0.076 mm and are made in tube diameters of about 14.5 mm to 203.2 mm. These casings are hereinafter referred to simply as xe2x80x9ccellulose casingxe2x80x9d.
Cellulose casing is most commonly produced by the well known and so called xe2x80x9cviscose processxe2x80x9d wherein viscose, a soluble cellulose derivative, is extruded as a tubular film. The annular extrusion die extends through the bottom of a coagulating and regenerating bath so that the extrusion proceeds in an upward direction through the bath. By extruding upwardly directly into the regenerating bath, the bath liquid supports the extrusion which is fragile and not self supporting during the initial phase of regeneration. After sufficient regeneration to be self supporting, the tube undergoes additional processing steps and is then washed, plasticized with glycerine or other polyol, and dried. Drying usually is accomplished while the tube is inflated with air at a pressure sufficient both to maintain a constant tube diameter and to orient the film.
The present invention involves an alternate cellulose production method in which a cellulose solution is formed by means of a simple dissolution rather than: requiring the formation of a cellulose derivative prior to forming a soluble substance (as in the viscose process). A cellulose dissolution process is described, for example, in U.S. Pat. No. 2,179,181 wherein a natural cellulose is dissolved by a tertiary amine N-oxide to produce solutions of relatively low solids content. The cellulose in the resulting solution is xe2x80x9cnonderivatizedxe2x80x9d in that the natural cellulose was not chemically reacted prior to dissolution to produce a soluble cellulose derivative as would occur for example, in the viscose process. U.S. Pat. No. 3,447,939 discloses use of N-methyl-morpholine-N-oxide (NMMO) as the tertiary amine N-oxide solvent wherein the resulting solutions, while having a low solids content, nevertheless can be used in chemical reactions involving the dissolved compound, or to precipitate the cellulose to form a film or filament.
More recent patents such as U.S. Pat. Nos. 4,145,532 and 4,426,288 improve upon the teachings of the ""939 Patent.
U.S. Pat. No. 5,277,857 discloses a method and apparatus for manufacturing cellulose food casing from a solution comprising nonderivatized cellulose, NMMO and water.
In the ""857 Patent, nonderivatized cellulose in a molten state, and contrary to the viscose process, is extruded as a tubular film downwardly through an air space and into a non solvent liquid such as a water bath. In the water bath, the NMMO solvent is removed to regenerate or precipitate the nonderivatized cellulose which is then washed of residual solvent plasticized and dried.
In U.S. Pat. No. 5,451,364 the manufacturing method as disclosed in the prior ""857 Patent is improved by the discovery that extruding the thermoplastic cellulose solution through a long air gap improves the properties of the resulting tubular cellulose film. In particular, the ""364 Patent discloses that the air gap should be over 152.4 mm and preferably from 152.4 mm to 304.88 mm long and perhaps longer.
Both the ""364 and ""857 Patents further disclose the use of a mandrel which depends from the extrusion die and about which the thermoplastic cellulose solution is extruded. This mandrel extends through the air gap and has its lower end face disposed below the level of the non solvent liquid bath. The mandrel for most of its, length is a slender shaft. The lower portion, however, is larger in diameter and is as large as, or larger than, the extruded tube diameter so it contacts around the entire inner circumference of the extruded tube. The mandrel shaft, being smaller in diameter, is radially spaced from the inner surface of the extruded tube.
The large diameter lower portion of the mandrel serves to size the tube as it enters the bath. Also, since it contacts the extruded tube, the enlarged lower portion of the mandrel together with the extrusion die provide spaced bearing points for stabilizing the extruded tube and preventing it from wandering.
The mandrel also is used to introduce a non solvent liquid into the interior of the extruded tube. One function of this introduced non solvent liquid, among others, is to lubricate around the circumference of the lower portion of the mandrel to prevent the extruded tube from binding as it passes over the surface of the lower portion or blocking when it later is collapsed to a flat width.
In this regard, a non solvent liquid or xe2x80x9cinner bathxe2x80x9d is introduced inside the extruded tube through ports in the mandrel shaft. This inner bath flows down the mandrel and pools where the extruded tube meets the enlarged lower end of the mandrel. This pooling distributes the non solvent around the mandrel so the entire outer circumference of the mandrel lower portion is wetted. Non solvent liquid then flows off the mandrel and to the bath within the extruded tube.
U.S. Pat. No. 5,766,540 discloses a mandrel structure allowing extrusion through even longer air gaps of up to 500 mm or more.
U.S. Pat. No. 5,759,478 discloses that certain properties of the cellulose film formed by the tubular extrusion as described in the ""857 Patent are enhanced by increasing the length of the enlarged lower or xe2x80x9csizing portionxe2x80x9d of the mandrel. It is speculated that maintaining the extruded tube in a stretched condition for a longer time by keeping it in contact with the sizing portion of the mandrel allows desirable orientation characteristics of the gel tube to xe2x80x9csetxe2x80x9d during the solvent extraction process: Whatever the reason, a longer contact time with the sizing portion of the mandrel was desirable and the ""478 Patent indicates that a preferred length of the sizing portion is about 50 mm.
However, as disclosed in the ""478 Patent, increasing the length of the sizing portion of the mandrel gives rise to another problem. This involves the removal of gas bubbles from the interface between the surface of the sizing portion and the inside surface of the extruded tube that likely are formed by out gassing of air from the extruded tube. In the ""478 Patent, these bubbles are removed through a series of interconnected circumferential grooves formed in the surface of the sizing portion.
It now has been found that both the stability of the extrusion process and properties of the casing are further improved when the length of the sizing portion is increased to lengths greater than the 50 mm disclosed in the ""478 Patent. Increasing the length of the sizing portion necessitated additional circumferential grooves to provide for the removal of gas bubbles. However, adding more grooves made difficult the drawing of the leading end of the extrusion over the sizing portion on the start-up of extrusion. This is because the increase in the number of grooves increased the likelihood of the leading end of the extrusion snagging on the edge of a groove.
Also, contrary to the disclosure of the ""478 Patent, the additional grooves did not act as air bearings to facilitate the passage of the extrusion over the sizing portion. Instead, each additional groove added an amount of frictional force so that the total drag caused by the extrusion passing over the many circumferential grooves was not compatible with the smooth passage of the extrusion over the mandrel sizing portion. If there are too many grooves, the frictional force may become so great that the extrusion breaks rather than being pulled over the mandrel. Accordingly, the addition of grooves to provide for gas removal as taught by the ""478 Patent limited the mandrel length and interfered with the benefits that were seen from the use of a mandrel longer than 50 mm. Thus, while it was found that mandrels of up to 152 mm or more were desirable from the standpoint of the stability of the extrusion process and casing properties, the additional circumferential grooves needed to accommodate gas removal over this length prevented use of the more desirable longer length mandrel.
Accordingly, it is an object of the present invention to provide an improved apparatus and method for extruding a seamless tube of a thermoplastic. nonderivatized cellulose solution to permit the use of a mandrel having a longer sizing portion to diametrically expand the extruded tube including means to accommodate and remove gas bubbles that are generated at the interface between the surface of the sizing portion and the inner surface of the extruded tube.
In the present invention, a tube of thermoplastic nonderivatized cellulose is downwardly extruded through a long air gap and into a bath of non solvent liquid as is generally disclosed in U.S. Pat. Nos. 5,277,857 and 5,451,364:
For purposes of this specification xe2x80x9cnonderivatizedxe2x80x9d cellulose means a cellulose which has not been subjected to covalent bonding with a solvent or reagent but which has been dissolved by association with a solvent or reagent through Van der Waals forces such as hydrogen bonding. xe2x80x9cNon solventxe2x80x9d means a liquid which is not a cellulose solvent.
As shown in the ""857 and ""364 Patents, extrusion is about a mandrel which depends from the extrusion die. The mandrel has a shaft portion and a lower end portion which is larger in diameter than the shaft. The lower end or xe2x80x9csizing portionxe2x80x9d of the mandrel has a diameter larger than the extruded diameter of the thermoplastic tube so the tube is stretched as it passes over the sizing portion. As noted above, gas bubbles tend to generate at the interface between the outer surface of the sizing portion and the inside surface of the extruded tube. In the present invention, the sizing portion is hollow for most of its length and openings in the outer surface of the sizing portion provide passages to conduct these bubbles directly from the surface to the hollow interior of the sizing portion. A conduit extending up from the hollow interior and through the shaft portion provides for the removal of gas from the hollow interior of the sizing portion.
Accordingly, the invention may be characterized in one aspect thereof by an apparatus for extruding a seamless tubular film from a thermoplastic solution composed of nonderivatized cellulose, a tertiary amine oxide cellulose solvent and water by extruding a tube of the solution downwardly from an annular die, through an air space and into an outer bath of a non solvent liquid for regenerating the cellulose from the solution, the extrusion being about a mandrel comprising. a shaft which depends from the die and extends through the air space and a sizing portion at a lower end of the shaft which is disposed for the majority of its length below the level of the liquid in the bath.
The sizing portion is formed by an annular wall that defines a hollow. cylinder having a length of from 60 to 152.4 mm or more and an outside diameter greater than the diameter of the tubular extrusion. The outer surface of the sizing portion is smooth and is provided with a plurality of openings that extend through the annular wall to the hollow interior of the cylinder. These openings allow gas bubbles generated at the interface between the outer surface of the sizing portion and the inner surface of the extrusion to pass through the annular wall directly into the hollow interior of the sizing portion. Preferably, the openings are arranged such that any drag created as the extrusion passes over the openings does not create a moment of force tending to turn or twist the extrusion with respect to the sizing portion.
From the interior of the sizing portion, the gas is removed upwardly through a conduit in the stem that opens into the hollow cylinder.
In another aspect, the present method may be characterized by a method of:
a) forming a seamless cellulose tube by downwardly extruding a tube of thermoplastic solution composed of non derivatized cellulose, a tertiary amine oxide and water through an air gap and into a non solvent bath;
b) during the course of extruding, causing the extrusion to pass over and about a vertically oriented mandrel having a sizing portion generally in the shape of a hollow cylinder that has a diameter larger than the extruded tube and a length of 60 to 152.4 mm or more, the mandrel having a smooth surface provided with a plurality of openings to the interior of the hollow cylinder;
c) passing gas bubbles evolving from the extruded tube at the interface between the extruded tube and sizing portion through the openings and into the hollow interior; and
d) removing the gas from the hollow interior upwardly through the mandrel.
Other objects and advantages of this invention will be apparent from the ensuing detailed disclosure and appended claims.